Edge-provided tool and method for the manufacture thereof

ABSTRACT

The invention relates to an edge-provided tool, such as a doctor blade, for use as wiping tool in the production of pulp and/or paper in different stages of the production process. It consists of an edge-provided strip of steel, the edge portion of which has been provided with a wear-resistant coating applied by means of laser technique, so that metallurgical binding is present between said coating and the steel strip, whereby a level of surface hardness of more than 850 UV being attained. Thereby, wear-resistant doctor blades having a long service life have been obtained.

Scraper or cleaning blades denominated “doctor blades” are widely usedin the paper and pulp industry for different purposes, but in most casesthey have the function of cleaning or scraping off material orleft-overs from the surface of a rotary roller. Such doctor blades have,for instance, the purpose of detaching a paper web from a stiff dryingcylinder by scraping the surface of the cylinder.

The paper pulp contains filler such as silicon dioxide, great particlesand paper fibers that wear out the doctor blade, the wear beingsubstantially evenly distributed along the edge, and wherein it holdsthat the rate of wear is greater than the rate of corrosion.Furthermore, the doctor blade is seen as an article of consumptionhaving very short service life, usually considerably smaller than oneday. In many cases, it is exchanged already after approximately 5 hours.Usually no regrinding of the doctor blade is carried out but it isdiscarded when consumed.

There are different principles in the paper mills for the use of doctorblades, with the same in general being worn-in initially for theformation of an edge. When the same has been formed, the wear isrelatively even. The proper “in-situ”-grinding-in is assumed to have aquality-influencing effect on the manufactured paper. If the doctorblade is used in a later stage of the production process of paper, itinfluences the paper quality and the surface structure of the paper, andtherefore it is seen as a very critical parameter. The doctor blades maybe of different length and vary between about 1 and 10 m.

Thus, since a doctor blade is subjected to extensive wear, differenttechniques are used in order to extend the service life thereof, such asaddition of wear-resistant material on the part of the blade that meetsthe cylinder. As an example of such a reinforcement of the top or edgesurface of the blade, ceramic hard coatings are presently used as apractical solution to bring down the blade wear. Such ceramic coatingsare usually applied to a doctor blade made by hardened and annealedcarbon steel and may be applied by thermal spraying, such as plasmaspraying or plasma depositing.

Examples of techniques to provide such wear-resistant coatings oncleaning or scraper blades of the type doctor blades (in English “doctorblade”) are found in GB-A-978 988, GB-A-1 289 609 and GB-A-2 130 924.The entire known technique is aimed at the provision of a wear-resistantcoating on the part of the blade that comes into contact with thesurface of a rotary cylinder.

According to prior art, with the purpose of improving the properties ofthe edge-provided tool, service life and thereby influence the paperquality, thermal (warm) spraying with HVOF (“High Velocity Oxygen Fuel”)is, among other things, used. The method gives sintering of the additivematerial and so-called diffusion binding at about 1000° C. Problems withthis coating method are cracks, pores, poor binding strength, adhesionproblems, etc. Materials sprayed today are ceramics, such as Al₂O₃.

Against this background, it is an object of the present invention to beable to provide an edge-provided tool, such as a doctor blade, whichtool has been provided with a coating having better adhesion than thatof hitherto known tools. Simultaneously, deformations of the toolarising otherwise should be tried to be minimized, at the same time asdesired fineness requirements and absence of scratches on the completedpaper product should be possible to meet. Furthermore, the supply ofenergy or influence of heat on the base material made by the surfacetreatment should as far as possible be minimized.

An additional object of the present invention is to produce a scraperand/or cleaning blade of primarily doctor blade type, which possesseslong service life and which in the paper manufacture imparts the paper asmooth and fine surface, free from scratches and other defects.

These and additional objects have in a surprising way succeeded to beattained by providing the tool with the features defined in theindependent claims. Preferred embodiments of the present invention aredefined in the dependent claims.

The invention will be closer described below in connection with thedrawing figures, where

FIG. 1 shows a perspective view of a doctor blade applied in directconnection to a roller so as to wiping off the paper pulp in theproduction of paper;

FIG. 2 shows an explanatory sketch of an edge-treated doctor bladeaccording to the present invention;

FIG. 3 shows an explanatory sketch of a coated doctor blade edgeaccording to the invention, seen from the side;

FIG. 4 shows an explanatory sketch of a laser-impregnated doctor bladeedge according to the invention, seen from the side; and

FIG. 5 shows an explanatory sketch showing laser coating of a doctorblade according to the invention.

Thus, the present invention relates to an edge-provided tool as well asa method for the manufacture thereof. More closely defined, theinvention relates to a type of tool that is called “doctor blade” andthat is a wiping, scraper and/or cleaning tool used in, for instance,the manufacture of paper, in order to get a smooth and fine paperproduct free from scratches and other defects. Also tools that usuallyare called coater blades, and also general knives, are comprised in thepresent invention. However, with the purpose of facilitating thedescription of the invention, the subject of the invention will below bedesignated “doctor blade”, in which concept also other denominations areintended to be comprised.

Thus, FIG. 1 shows a doctor blade 1, which abuts against a roller 2 and,for instance, scrapes away pulp from the same. As a preferred example ofmaterials that have turned out to work well in order to coat the edgeportion of the material by means of laser technique according to theinvention, steel grades having a hardness of at least 450 HV have beenselected. An example of such a steel is a carbon steel having acomposition comprising (in % by weight) 0.8-1.2% of C, preferably about1% of C, 0.20-0.35% of Si, 0.35-0.50% of Mn, maximum 0.02% of P, maximum0.01% of S, with Fe as balance and the content of some additionalelement in the periodic system in contents below 0.5%.

FIGS. 2 and 3 show a steel strip 3, which on the edge thereof has beencoated with a coating 4 by laser technique in accordance with theinvention. Thereby, the resistance to wear is increased considerably.The coating is composed of, for instance, aluminum oxide or stellite(for instance stellite 12). The laser technique is, per se, well knownto a person skilled in the art and is visualized in FIG. 5. In the lasercoating, the atomic (also called “metallurgical”) binding to the steelsubstrate characteristic of the invention is attained, which isvisualized in FIG. 3.

FIG. 4 shows a doctor blade in cross-section, which has been coatedwith, for instance, TiC in accordance with the present invention. Thelaser impregnation technique is described in the patent WO 99/56906. Thesteel strip is designated 5 and the impregnation applied by means oflaser technique is designated 6. Also in this figure it is visualizedthat the particles/the carbides have penetrated into the steelsubstrate, whereby the limit between the two to a significant extenthaving been erased. The coating has been effected with, for instance,aluminum oxide or stellite, such as in the FIGS. 2 and 3, while theimpregnation is carried out with suitable carbides and/or nitrides.

According to FIG. 5, the surface-reinforcing portion in the formed edgeportion of a strip-shaped doctor blade is provided in the way that, bymeans of laser technique, a coating, supplied by means of powdermaterial 7, is applied to the edge under such supply of heat that thepowder is fused with the basic material in the doctor blade, so thatatomic and metallurgical binding arise. The supply of heat takes placeby means of a laser gun 8, a coating 9 being provided on the substrate.The supplied material in the flow of powder 7 suitably consists of arefractory material, such as aluminum oxide. Alternatively, a materialsuch as stellite may be used. In connection with the coating 9 havingbeen applied to the doctor blade, it is important that the material canundergo a quick cooling, so that a desired fine structure is obtained,which is characterized by both toughness and hardness, more closelydefined to a level of hardness corresponding to 850-1300 HV. In thisway, simultaneously high bearing strength in the surface layer isattained at the same time as requirements of low friction and desiredcorrosion resistance can be fulfilled.

As has been mentioned above, the coating may essentially be composed ofaluminum oxide or stellite. However, the coating may also contain or becomposed of other refractory materials, such as metallic oxides,metallic silicates, metallic carbides, metallic borides, metallicnitrides and mixtures thereof. Especially preferred ceramic materialsare selected among aluminum oxide, chromic oxide, zirconium oxide,wolfram carbide, chromium carbide, zirconium carbide, tantalum carbide,titanium carbide, titanium nitride, niobium carbide and borides.

As has been mentioned above, a laser impregnation may also be carriedout by, for instance, carbides and nitrides such as TiC, NbC and TiNbeing added to the surface of the base material during the lasertreatment. The particles are added by spraying under high gas pressureat the same time as the laser beam locally melts the surface layer onthe material so that the wear-resistant particles can penetrate into thesubstrate. Furthermore, the laser coating may be effected in a pluralityof rounds, so that multiple layers are obtained. The steel strip thenreceives a very wear-resistant surface.

The thickness of the steel strip is the normal one for doctor blades inthe paper industry and may vary between 0.2 and 3 mm, suitably between0.305 and 1.27 mm. The thickness of the coating or the impregnation maysuitably be between 5 and 15% of the thickness of the steel strip.

1. Doctor or coater blade, in particular for use as wiping, scraperand/or cleaning tool in production of paper pulp and/or paper indifferent stages of the production process, comprising an edge-providedstrip of steel, the edge portion of which has been provided with awear-resistant coating, wherein the edge portion of the blade is coatedwith surface-reinforcing coating applied by laser technique, so that themetallurgical binding is present between said wear-resistant coating andthe steel strip, and the edge portion has a hardness of more than 1000HV.
 2. Doctor or coater blade according to claim 1, wherein thewear-resistant coating has a level of surface hardness of at least 850HV.
 3. Doctor or coater blade according to claim 1, wherein the edgeportion of the blade has a surface layer applied by laser coating, thethickness of which layer constitutes 5-15% of the thickness of theblade.
 4. Doctor or coater blade according to claim 1, wherein the edgeportion of the blade has a surface-reinforcing portion applied by lasercoating or laser impregnation, the thickness of which portionconstitutes 5-15% of the thickness of the blade.
 5. Doctor or coaterblade according to claim 1 wherein the steel is a carbon steel with achemical composition in % by weight being 0.8-1.2% of C, preferablyabout 1% of C, 0.20-0.35% of Si, 0.35-0.50% of Mn, maximum 0.02% of P,maximum 0.01% of S, with Fe as balance and the content of someadditional element in the periodic system in contents below 0.5%. 6.Method for the manufacture of a doctor or coater blade according toclaim 1 wherein a material manufactured from steel is first rolled outand edge-treated to have an edge portion formed along one of the edges,said edge portion then is provided with a surface-reinforcing layerapplied by laser technique, in such a way that a metallugical bindingarises between said layer and the subjacent steel substrate, and theedge treatment is provided in the way that the steel substrate issubjected to a laser treatment during supply of powder at such a supplyof heat that the powder is fused with the steel substrate while formingan atomic/metallurgical binding.
 7. Method for the manufacture of adoctor or coater blade according to claim 1 wherein a materialmanufactured from steel is first rolled out and edge-treated to have anedge portion formed along one of the edges, said edge portion then isprovided with a surface-reinforcing layer applied by laser technique, insuch a way that a metallugical binding arises between said layer and thesubjacent steel substrate, and the edge treatment is provided in the waythat the steel substrate is subjected to a laser impreganation andrecovery, material particles of a ceramic material penetrating into thesurface melt by laser, so that an atomic/metallurgical binding arises.8. Method according to claim 6, wherein the supplied powder essentiallycontains aluminum oxide.
 9. Method according to claim 6, wherein thesupplied material essentially contains stellite.
 10. Method according toclaim 7, wherein the impregnation is carried out in the way thatcarbides and nitrides, such as TiC, NbC and/or TiN, are supplied to thesteel substrate.
 11. Method according to claim 6, wherein the chemicalcomposition of the steel in % by weight is 0.8-1.2% of C, preferablyabout 1% of C, 0.20-0.35% of Si, 0.35-0.50% of Mn, maximum 0.02% of P,maximum 0.01% of S, with Fe as balance and the content of someadditional element in the periodic system in contents below 0.5%. 12.Method according to claim 6, wherein the wear-resistant coating has alevel of surface hardness of more than 850 HV.
 13. Doctor or coaterblade according to claim 2, wherein the steel is a carbon steel with achemical composition in % by weight being 0.8-1.2% of C, preferablyabout 1% of C, 0.20-0.35% of Si, 0.35-0.50% of Mn, maximum 0.02% of P,maximum 0.01% of S, with Fe as balance and the content of someadditional element in the periodic system in contents below 0.5%. 14.Doctor or coater blade according to claim 3, wherein the steel is acarbon steel with a chemical composition in % by weight being 0.8-1.2%of C, preferably about 1% of C, 0.20-0.35% of Si, 0.35-0.50% of Mn,maximum 0.02% of P, maximum 0.01% of S, with Fe as balance and thecontent of some additional element in the periodic system in contentsbelow 0.5%.
 15. Doctor or coater blade according to claim 4, wherein thesteel is a carbon steel with a chemical composition in % by weight being0.8-1.2% of C, preferably about 1% of C, 0.20-0.35% of Si, 0.35-0.50% ofMn, maximum 0.02% of P, maximum 0.01% of S, with Fe as balance and thecontent of some additional element in the periodic system in contentsbelow 0.5%.
 16. Method according to claim 7, wherein the supplied powderessentially contains aluminum oxide.
 17. Method according to claim 7,wherein the supplied material essentially contains stellite.
 18. Methodaccording to claim 7, wherein the chemical composition of the steel in %by weight is 0.8-1.2% of C, preferably about 1% of C, 0.20-0.35% of Si,0.35-0.50% of Mn, maximum 0.02% of P, maximum 0.01% of S, with Fe asbalance and the content of some additional element in the periodicsystem in contents below 0.5%.
 19. Method according to claim 7, whereinthe wear-resistant coating has a level of surface hardness of more than850 HV.
 20. Method according to claim 16, wherein the impregnation iscarried out in the way that carbides and nitrides, such as TiC, NbCand/or TiN, are supplied to the steel substrate.